Method of metallizing the screen of a cathode ray tube



May 14, 1957 s. LEVY 2,792,315

METHOD OF METALLIZING THE SCREEN OF A CATHODE RAY TUBE! Filed Dec. 11,1955 'INVENTOR SIDNEY LEVY METHOD F METALLIZING THE SCREEN OF A CATHODERAY TUBE Sidney Levy, Garden City, N. Y., assignor to Sylvania ElectricProducts Inc, a corporation of Massachusetts Application December 11,1953, Serial No. 397,695

11 Claims. (Cl. 117-335) This invention relates to a luminescentphosphor screen. More particularly it relates to an improved method ofmetallizing the screen of a cathode ray tube.

The light output and resolution of a cathode ray tube is materiallyincreased when an electron pervious light reflecting metallic film isapplied to the beam side of its luminescent screen. This fact has beenknown for some time and consequently those cathode ray tubes which havebeen used for projection television have been pro vided with such ametallized screen. In order that the screen can give optimum results ithas been said that the metallized film should be thin enough to causenegligible absorption of the electron beam, should be opaque, should berelatively smooth and highly light reflecting and should also havesuificient conductivity to conduct the full beam current. Furthermore,it should be strong enough to withstand the stresses due to the effectof the focused beam, should be durable enough to withstand theprocessing of the cathode ray tube and finally should not reactchemically with the luminescent materials forming the screen.

in order that the metallized screen could be made opaque, relativelysmooth, and highly reflecting, it has been found necessary to cover theluminescent phosphor with a thin film of organic material whichpresumably is stretched over the depressions in the phosphor deposit soas to present a surface which is more nearly plane. This organic film isthen covered with a film of metal, usually aluminum which is depositedthereon by a vaporization technique. Although the presence of theorganic film prior to the metallizing step might seem to be highlyquestionable it has been found that Without this organic film the metalfilm does not possess the necessary light reflecting property and thatthe brightness is reduced to as low as /3 of the value obtained when anorganic film is present.

In accordance with the established methods of the prior art a lacquerfilm is usually applied by spreading a collodion solution on a watersurface overlaying the phos phor deposit allowing the lacquer to spreadoutwardly over the entire surface until the solvent evaporates leaving athin film of collodion (nitrate of cellulose) behind; thereupon thewater beneath the lacquer film is decanted and the lacquer film isallowed to drop down into intimate contact with the phosphorescentscreen below. The depth of the water level varies somewhat dependentupon the curvature of the face-plate of the picture tube. However, it isusually at least a half inch at the shallowest point. After the waterhas been allowed to flow out from underneath the film and the film hasbeen allowed to adhere to the powder deposit, the tube is dried in acurrent of warm air and the film stretches over the microscopicdepressions and gaps in the luminescent powder deposit.

This method of applying a lacquer film gives rise to a considerableproportion of rejects in production inasmuch as the film is mechanicallyweak; further subject to tearing and the spreading and adherence of thefilm are affected by the conditions of the interior surface of a cathoderay tube and the surface of the phosphor.

It isan object of this invention to provide a method of coating thephosphor screen with an organic film which is not dependent upon arelatively deep cushioning lacquer.

It is a further object of this invention to provide a method of applyingan organic film over the phosphor screen in such manner that any desiredthickness and hence mechanical strength of the film may be attained.

It is a still further object of this invention to provide a method whichwill insure complete coverage of the phosphor screen by the appliedorganic film.

It is a still further objectof this invention to provide a method whichwill increase the uniformity of the film over its entire surface area.

These objects and other advantages incidental thereto can be obtained inaccordance with this invention by spray ing a film forming material,such as collodion, in a suitable solvent either directly onto aluminescent screen or else spraying this film forming material onto aphosphor screen which is still wet but not necessarily covered to anysubstantial degree with a water layer.

These and other objects, features and advantages of this invention willbecome apparent when considered in view of the following description andthe accompanying drawings wherein:

Figure 1 illustrates a particular step in the practice of thisinvention, and

Figures 2 and 3 are enlarged fragmentary sectional views showing afluorescent cathode ray tube screen in successive stages of treatment.

Referring particularly to Figure 2 there is shown an enlarged bulbousportion of the envelope 1d of a television picture tube with the bottomor face portion of the envelope 10 covered with a screen or layer ofphosphor material 12. This layer in the illustrated case shown in thedrawing is at the stage obtained after decantation and drying of thephosphor screen material. As shown in the drawings the spray nozzle 20projects downwardly through the neck portion of the enevlope and ispositioned a sufiicient distance from the face of the envelope to permitthe emulsion to be sprayed in such manner as to cover the entire faceplate of the envelope as shown a at 22 in the drawing.

The emulsion which is to be used for this purpose can be any of the wellknown organic heat decomposible film forming materials now used in theprior art for this purpose, such as, for example, nitro-celluloselacquer, flexible collodion, or parlodion. The material is emulsified inwater, the amounts and the proportions of the ingredients kept Withinthe limit which will cause a thin water film to be produced and coverthe phosphor layer when the emulsion is sprayed thereon and leave a filmof lacquer over this water layer. In order to pro mote or improve thewetting characteristics of the phosphor it has been found desirable insome instances to incorporate a small amount of potassium silicate inthe water phase of the emulsion.

When an emulsion of the type hereinabove described is used as the spraylacquer to cover the dry phosphor complete coverage is assured which isnot the case when the water spreading technique of the prior art is usedfor when the water spreading technique is used in a large tube thesolvent often evaporates and the lacquer or film sets before thesolution had a chance to spread to the wall of the tube. This isoccasioned by the fact that as the solvent evaporates the viscosityincreases rapidly and at the edges of the spreading film there may besufficient evaporation to form a rigid film which will resist fuptherspreading. Furthermore, in the case of the water spreading techniqueunless the film stretches during the decantation the latter stages ofthe decantation of the film will not cover the entire phosphor surface.

When the film is. to be formed in accordance with this invention thephosphor layer can, of course, be formed on the face of the tube inaccordance with the 'usual practice such as, for example, by thesettling process from a liquid suspensiom After the phosphor has beensettled onto the face of the tube and the supernatant liquid hasbeendecanted the phosphor screen is dried. An emulsion is formed bydiluting one volume of a nitrocellulose solution which has beenformulated, for example, by mixing the following ingredients in theproportions given: 7.5% cellulose nitrate, 1.5% glycol sebacate, 26.2%ethyl acetate, 19.6% butyl acetate, 7.8% butyl alcohol and 36.4%aromatic petroleum naphtha with four volumes of amyl acetate. Onemilliliter of dioctyl phthallate is added per liter of solution. Onevolume of this solution is then emulsified with nine volumes of water ina blender. The Water contains one half percent by weight of potassiumsilicate. This emul sion is then sprayed onto the face of the driedphosphor screen. As the emulsion hits the dried phosphor it breaks upand the water tends to wet the phosphor particles leaving the organicfilm forming material to float on top of the very thin layer of water.As the solvent evaporates the film hardens whereupon the sprayed tube issent through an oven and the water beneath the film is evaporatedleaving the phosphor covered with a thin uniform coating of organic filmwhich presents a smooth plane surface ready for further processing toform a metallized film by a vaporization technique for example. It is,of course, understood that the nitrocellulose solution described aboveis merely illustrative and is not to be considered as critical.

The spraying of the emulsion may be used on a dried phosphor screenWithout previously wetting the screen or providing a water cushion aswould be required previous to spraying lacquer solution. Certainscreening processes produce phosphor screens which must be dried inorder to achieve adherence of the phosphor to the glass. It would be anadvantage therefore to be able to spray emulsion directly onto suchscreens without intermediary steps.

Control of the thickness and uniformity of the lacquer film is moreconvenient with the emulsion than with the solution because of thegreater volume which must be sprayed for the same organic filmthickness. Also, the size of the droplets of lacquer is determined bythe emulsification conditions and not by the spraying conditions, andis, therefore, uniform throughout the spray.

While the above description and drawings submitted herewith disclose apreferred and practical embodiment of the luminescent screen of thisinvention it will be understood that the specific details ofconstruction and arrangement of parts as shown and described are by wayof illustration and are not to be construed as limiting the scope of theinvention.

What is claimed is:

1. In the method of metallizing the screen of a cathode ray tubeincluding the steps of settling a phosphor screen from a settlingsolution onto the screen-forming surface of the cathode ray tube,vaporizing a metallic coating onto said phosphor screen, and baking thetube, the -improvement in preparing said phosphor screen for theapplication of said metallic coating which comprises spraying anemulsion of an organic heat decomposable film forming material onto saidphosphor screen to form a coating, and drying the coating so formed topresent a surface over said phosphor screen which is to receive saidmetallic coating.

2. In the method of metallizing the screen of a cathode ray tubeincluding the steps of settling a phosphor screen from a settlingsolution onto the screen forming surface of the cathode ray tube,vaporizing a metallic coating 4 onto said phosphor screen and baking thetube, the improvement in preparing said phosphor screen for the ap"-plication of said metallic coating which comprises spraying an emulsionof an organic heat decomposable film forming material having anitrocellulose base onto said phosphor screen to form a coating, anddrying the coating so formed to present a surface over said phosphorscreen which is to receive said metallic coating.

3. In the method of aluminizing the screen of a cathode ray tubeincluding the steps of settling a phosphor screen from a settlingsolution onto the screen forming surface of the cathode ray tube,vaporizing an aluminum coating onto said phosphor screen, and baking thetube, the

improvement in preparing said phosphor screen for the application ofsaid aluminum coating which comprises spraying an emulsion of an organicheat decomposable film forming material onto said phosphor screen toform a coating, and drying the coating so formed to present a surfaceover said phosphor screen which is to receive said aluminum coating.

4. In the application of a reflective metallic coating to a fluorescentscreen of a cathode ray tube, the improvement comprising the preparationof said fluorescent screen to receive said reflective metallic coatingwhich includes the steps of spraying an emulsion of an organic heatdecomposable film forming material and water onto said fluorescentscreen, the spraying being effective to cause the emulsion to produce awater film covering said fluorescent screen and an organic heatdecomposable film over the water film, and drying said organic film toform a smooth organic coating over said fluorescent screen which is toreceive said reflective metallic coating.

5. In the application of a reflective metallic coating to afluorescentscreen of a cathode ray tube, the improvement comprising the preparationof said fluorescent screen to receive said reflective metallic coatingwhich includes the steps of spraying an emulsion of an organic heatdecomposable film forming material and water onto said fluorescentscreen, the spraying being effective to cause the emulsion to produce awater film covering said fluorescent screen and an organic heatdecomposable film over the water film, drying said organic film to forma smooth organic coating over said fluorescent screen which is toreceive said reflective metallic coating, and after applying saidreflective metallic coating, baking said tube to drive otf residual filmforming material.

6. In the application of a reflective metallic coating to a fluorescentscreen of a cathode ray tube, the improvement comprising the preparationof said fluorescent screen to receive said reflective metallic coatingwhich includes the steps of spraying an emulsion of an organic heatdecomposable film forming material and water onto said fluorescentscreen, the emulsion having potassium silicate in the water phase andthe spraying being effective to cause the emulsion to produce a waterfilm covering said fluorescent screen and an organic heat decomposablefilm over the water film, and drying said organic film to form a smoothorganic coating over said fluorescent screen which is to receive saidreflective metallic coating.

7. In the application of a reflective metallic coating to a fluorescentscreen of a cathode ray tube, the improvement comprising the preparationof said fluorescent screen to receive said reflective metallic coatingwhich includes the steps of spraying an emulsion of an organic heatdecomposable film forming material having a nitrocellulose base andwater onto said fluorescent screen, the spraying being effective tocause the emulsion to produce a water film covering said fluorescentscreen and an organic heat decomposable film over the water film, anddrying said organic film to form a smooth organic coating over saidfluorescent screen which is to receive said reflective metallic coating.

8. In the manufacture of a cathode ray tube including the steps ofsettling a phosphor screen from a settling solution onto the screenforming surface of the cathode ray tube, decanting the settlingsolution, and applying a reflective metallic coating over said phosphorscreen, the improvement in preparing said phosphor screen for theapplication of said reflective metallic coating which comprisespreparing an emulsion of water and an organic heat decomposable filmforming material, spraying said emulsion onto said phosphor screen suchthat said emulsion breaks up whereby the water tends to wet saidphosphor screen and said organic film forming material floats as a filmon top of the water in said emulsion, and hardening said film to presenta smooth planar surface over said phosphor screen which planar surfaceis to receive said reflective metallic coating.

9. In the manufacture of a cathode ray tube including the steps ofsettling a phosphor screen from a settling solution onto the screenforming surface of the cathode ray tube, decanting the settlingsolution, and applying a reflective metallic coating over said phosphorscreen, the improvement in preparing said phosphor screen for theapplication of said reflective metallic coating which comprisespreparing an emulsion of water and an organic heat decomposable filmforming material with the water phase of said emulsion containingpotassium silicate, spraying said emulsion onto said phosphor screensuch that said emulsion breaks up whereby the water tends to wet saidphosphor screen and said organic film forming material floats as a filmon top of the water in said emulsion, and hardening said film to presenta smooth planar surface over said phosphor screen which planar surfaceis to receive said reflective metallic coating.

10. In the manufacture of a cathode ray tube including the steps ofsettling a phosphor screen from a settling solution onto the screenforming surface of the cathode ray tube, decanting the settlingsolution, applying a reflective metallic coating over said phosphorscreen, the improvement in conditioning said phosphor screen for theapplication of said reflective metallic coating which comprisespreparing an emulsion of water, a solvent and an organic heatdecomposable film forming material, spraying said emulsion onto saidphosphor screen such that said emulsion breaks up whereby the watertends to wet said phosphor screen and said organic film forming materialfloats as a film on top of the water in said emulsion, and hardeningsaid film to present a smooth planar surface over said phosphor screenwhich planar surface is to receive said reflective metallic coating.

11. In the manufacture of a cathode ray tube including the steps ofsettling a phosphor screen from a settling solution onto the screenforming surface of the cathode ray tube, decanting the settlingsolution, applying a re- Elective metallic coating over said phosphorscreen, the improvement in preparing said phosphor screen for theapplication of said reflective metallic coating which comprisespreparing an emulsion of water and an organic heat decomposable filmforming material having a nitrocellulose base, spraying said emulsiononto said phosphor screen such that said emulsion breaks up whereby thewater tends to wet said phosphor screen and said organic film formingmaterial floats as a film on top of the water in said emulsion,hardening said film to present a smooth planar surface over saidphosphor screen which planar surface is to receive said reflectivemetallic coating, and after applying said reflective metallic coating,baking said tube to drive off residual film forming material.

References Cited in the file of this patent UNITED STATES PATENTS

1. IN THE METHOD OF METALLIZING THE SCREEN OF A CATHODE RAY TUBEINCLUDING THE STEPS OF SETTLING A PHOSPHOR SCREEN FROM A SETTLINGSOLUTION ONTO THE SCREEN-FORMING SURFACE OF THE CATHODE RAY TUBE,VAPORIZING A METALLIC COATING ONTO SAID PHOSPHOR SCREEN, AND BAKING THETUBE, THE IMPROVEMENT IN PREPARING SAID PHOSPHOR SCREEN FOR THEAPPLICATION OF SAID METALLIC COATING WHICH COMPRISES SPRAYING ANEMULSION OF AN ORGANIC HEAT DECOMPOSABLE FILM FORMING MATERIAL ONTO SAIDPHOSPHOR SCREEN TO FORM A COATING, AND DRYING THE COATING SO FORMED TOPRESENT A SURFACE OVER SAID PHOSPHOR SCREEN WHICH IS TO RECEIVE SAIDMETALLIC COATING.